European Journal of Clinical Nutrition (2014) 68, 1274 © 2014 Macmillan Publishers Limited All rights reserved 0954-3007/14 www.nature.com/ejcn
LETTER TO THE EDITOR
Response to: Is inverted BMI really better than BMI in predicting body fatness in children? European Journal of Clinical Nutrition (2014) 68, 1274; doi:10.1038/ ejcn.2014.147; published online 30 July 2014 We thank Dr Singhal1 for their interest in our article.2 The observations made by Dr Singhal are accurate in that inverted body mass index (iBMI) predicted 0.2% greater variance in body fatness than body mass index (BMI). However, we believe our assertion that iBMI is ‘better’ than BMI in our sample population is still valid and appropriate. This is for several reasons and not solely based on results from statistical analysis. Firstly, we would consider any increase in the prediction of variance in a variable to represent ‘better’ performance. This would hold even if the change in prediction is small, as in some instances even a small magnitude of improvement is practically important. Secondly, and more importantly, the reason why iBMI might be considered ‘better’ than BMI in children is because it addresses some of the key problems when using BMI as a proxy for fatness in children. In brief, the validity of BMI as a measure of adiposity has been based on the assumption that as BMI increases so does adiposity.3 However, the assumption of a linear body fat–BMI relationship has been questioned by studies reporting strong evidence of curvature in this association.4 This linear relationship is particularly not the case in children.5 If this curvature is ignored and linearity assumed, then the true relationship between body fatness and BMI will be systematically underestimated in people with lower values of BMI and overestimated in people with higher BMI values.6 Furthermore, BMI is also not normally distributed in children.6 Thus, when BMI is used in statistical analysis, as is often the case, assumptions of normality are violated and inferences made cannot be trusted.6 Thirdly, iBMI is based on sound biological principles (for a review see Nevill and Holder7). This is due to lean body mass being more stature related than body mass. Hence, iBMI was validated from a biological standpoint, whereas BMI is an
empirically generated model, not originally derived from any biological principles.7 It is for the above-mentioned reasons, taken collectively rather than simply from the outcome of statistical analysis, that we feel it is correct to assert that iBMI is a ‘better’ proxy for body fatness than BMI in children. CONFLICT OF INTEREST The author declares no conflict of interest.
MJ Duncan Department of Biomolecular and Sports Sciences, Coventry University, Coventry, UK E-mail: [email protected] REFERENCES 1 Singhal N. Is inverted BMI really better than BMI in predicting body fatness in children? Eur J Clin Nutr 2014; 68: 972. 2 Duncan MJ, Martins C, Silva G, Marques E, Mota J, Aires L. Inverted BMI rather than BMI is a better predictor of DEXA determined body fatness in children. Eur J Clin Nutr 2014; 68: 638–640. 3 Nevill AM, Metsios GS, Jackson AS, Wang J, Thornton J, Gallagher D. Can we use the Jackson and Pollock equations to predict body density/fat of obese individuals in the 21st century? Int J Body Comp Res 2008; 6: 115–122. 4 Rothman KJ. BMI-related errors in the measurement of obesity. Int J Obes 2008; 32 (Suppl 3): S56–S59. 5 Federico B, D’Aliesio F, Pane F, Capelli G, Rodio A. Body mass index has a curvilinear relationship with the percentage of body fat among children. BMC Res Notes 2011; 18: 301. 6 Nevill AM, Stavropoulos-Kalinoglou A, Metsios GS, Koutedakis Y, Holder RL, Kitas GD et al. Inverted BMI rather than BMI is a better proxy for percentage body fat. Ann Hum Biol 2011; 36: 681–684. 7 Nevill AM, Holder RL. Body mass index: a measure of fatness or leanness? Br J Nut 1995; 73: 507–516.
LETTER TO THE EDITOR
Response to: Is inverted BMI really better than BMI in predicting body fatness in children? European Journal of Clinical Nutrition (2014) 68, 1274; doi:10.1038/ ejcn.2014.147; published online 30 July 2014 We thank Dr Singhal1 for their interest in our article.2 The observations made by Dr Singhal are accurate in that inverted body mass index (iBMI) predicted 0.2% greater variance in body fatness than body mass index (BMI). However, we believe our assertion that iBMI is ‘better’ than BMI in our sample population is still valid and appropriate. This is for several reasons and not solely based on results from statistical analysis. Firstly, we would consider any increase in the prediction of variance in a variable to represent ‘better’ performance. This would hold even if the change in prediction is small, as in some instances even a small magnitude of improvement is practically important. Secondly, and more importantly, the reason why iBMI might be considered ‘better’ than BMI in children is because it addresses some of the key problems when using BMI as a proxy for fatness in children. In brief, the validity of BMI as a measure of adiposity has been based on the assumption that as BMI increases so does adiposity.3 However, the assumption of a linear body fat–BMI relationship has been questioned by studies reporting strong evidence of curvature in this association.4 This linear relationship is particularly not the case in children.5 If this curvature is ignored and linearity assumed, then the true relationship between body fatness and BMI will be systematically underestimated in people with lower values of BMI and overestimated in people with higher BMI values.6 Furthermore, BMI is also not normally distributed in children.6 Thus, when BMI is used in statistical analysis, as is often the case, assumptions of normality are violated and inferences made cannot be trusted.6 Thirdly, iBMI is based on sound biological principles (for a review see Nevill and Holder7). This is due to lean body mass being more stature related than body mass. Hence, iBMI was validated from a biological standpoint, whereas BMI is an
empirically generated model, not originally derived from any biological principles.7 It is for the above-mentioned reasons, taken collectively rather than simply from the outcome of statistical analysis, that we feel it is correct to assert that iBMI is a ‘better’ proxy for body fatness than BMI in children. CONFLICT OF INTEREST The author declares no conflict of interest.
MJ Duncan Department of Biomolecular and Sports Sciences, Coventry University, Coventry, UK E-mail: [email protected] REFERENCES 1 Singhal N. Is inverted BMI really better than BMI in predicting body fatness in children? Eur J Clin Nutr 2014; 68: 972. 2 Duncan MJ, Martins C, Silva G, Marques E, Mota J, Aires L. Inverted BMI rather than BMI is a better predictor of DEXA determined body fatness in children. Eur J Clin Nutr 2014; 68: 638–640. 3 Nevill AM, Metsios GS, Jackson AS, Wang J, Thornton J, Gallagher D. Can we use the Jackson and Pollock equations to predict body density/fat of obese individuals in the 21st century? Int J Body Comp Res 2008; 6: 115–122. 4 Rothman KJ. BMI-related errors in the measurement of obesity. Int J Obes 2008; 32 (Suppl 3): S56–S59. 5 Federico B, D’Aliesio F, Pane F, Capelli G, Rodio A. Body mass index has a curvilinear relationship with the percentage of body fat among children. BMC Res Notes 2011; 18: 301. 6 Nevill AM, Stavropoulos-Kalinoglou A, Metsios GS, Koutedakis Y, Holder RL, Kitas GD et al. Inverted BMI rather than BMI is a better proxy for percentage body fat. Ann Hum Biol 2011; 36: 681–684. 7 Nevill AM, Holder RL. Body mass index: a measure of fatness or leanness? Br J Nut 1995; 73: 507–516.
